Flame resistant, epoxy resin impregnated, glass cloth laminates have been used in the printed circuit industry to make high grade copper clad circuit boards. These high grade circuit boards must have excellent electrical properties, superior dimensional stability and mechanical strength, and outstanding chemical resistance. Moisture resistant laminates of this type are taught by Alvino et al., in U.S. Pat. No. 4,327,143. There, bisphenol A or epoxy novolac resins, are mixed with tetrabromobisphenol A, a flame retardant agent, at about 90.degree. C. After mixing and cooling, an epoxy diluent is added, followed by addition of a dicyandiamide derivative (N-cyano-N'-hydroxymethyl guanidine) used as a curing agent, to provide an impregnating resin. This resin is impregnated into glass cloth sheet material which could be used to make NEMA (National Electrical Manufacturers Association) FR-4 grade laminates.
While these laminates have excellent properties, the search continues for glass cloth and cellulosic paper based laminates having improved thermal aging characteristics, higher bond strength and heat distortion temperatures, i.e., Tg values, and even better blister resistance; and for "B"-staged prepregs, useful for manufacturing such laminates, having improved shelf life, and low levels of ionic contamination.
Pucci, in U.S. Pat. No. 4,311,753, attempted to solve problems of poor "B"-staged prepreg shelf life, and laminate solvent resistance, for glass cloth based prepregs and laminates useful in manufacturing flame resistant copper clad circuit boards. There, glass cloth "B"-staged prepregs are made, impregnated with a resin containing: a di and tetra functional epoxy mixture consisting of a brominated epoxy resin, such as a reacted mixture of bisphenol A epoxy and tetrabromobisphenol A, and a polyglycidyl ether of tetraphenylethane; dicyandiamide curing agent; a tetraalkylguanidine catalyst; and a solvent. Pucci et al., in U.S. Pat. No. 4,343,731, attempted to solve wetting properties of epxoy resins used in impregnating solutions for woven glass cloth. There, an epoxy resin is formulated from the fusion product of bisphenol A epoxy, quarternary phosphonium halide catalyzed bisphenol A epoxy, and tetrabromobisphenol A. This is used in combination with a dicyandiamide containing curing agent and solvent.
In the area of flame resistant paper based laminates for copper clad circuit board application, Nichols, in U.S. Pat. No. 3,804,693, attempted to improve room temperature shearing, blanking, punching, and other fabricating properties. There, a paper web is first impregnated with a water soluble phenolic resin mixed with a solvent, such as methyl alcohol. Then, the wetted paper web is impregnated with an epoxy modified phenolic resin comprising an admixture of: oil and/or rosin modified phenolic resin; water soluble phenolic resin; and bisphenol A epoxy resin; to which is added a brominated or chlorinated flame resistant additive, such as chlorinated bisphenyl or tris(2,3-dibromopropyl)phosphate, mixed with antimony oxide. The phenolic resins taught are of the resole type, where the mole ratio of (phenol):(formaldehyde) is from (1):(1 to 1.5).
Claybaker, in U.S. Pat. Nos. 4,043,954 and 4,254,187, makes flame resistant laminates from paper web sheets impregnated with an opoxy modified phenolic resin mixture containing: tung oil modified phenolic resole resin; epoxy resin prepared from a tetrabromobisphenol A; a flame retardant mixture of triphenyl phosphate and antimony trioxide; barium hydrate; low viscosity, catalyzed phenol-formaldehyde resin to facilitate paper penetration; and solvent.
Lee & Neville, in the Handbook of Epoxy Resins, McGraw-Hill, 1967, ch. 11-13 to 11-16, teach a wide variety of curing agents for epoxies, including resole phenolformaldehyde resins; and also solid, high molecular weight phenolic novolac resins for use with high melting point bisphenol A epoxy resins in solvent solution, when catalyzed with benzyldimethylamine.
Flame resistant composite laminates, made from a resin impregnated cellulose paper core bonded to resin impregnated woven glass cloth, are taught by Gause et al., in U.S. Pat. No. 3,895,158. There, resins comprising bisphenol A or novolac epoxy, in combination with chlorinated phenol or brominated epoxy flame retardants, and curing agents such as dicyandiamide, are used in the glass cloth layer. The cellulose paper layer is impregnated with the same epoxy resin but using an anhydride curing agent.
The improvements described in the prior art do not, however, completely solve "B"-staged prepreg shelf life problems, or problems associated with possible attack of nitrogen cross link junctions introduced by dicyandiamide type curing agents, upon thermal aging. Nor has there been a completely satisfactory solution to moisture resistance problems which also addresses bond strength problems and improved flexural strength upon thermal aging. Thus, there is a need for new and improved flame resistant "B"-staged prepregs for copper clad printed circuit boards and other applications.